Reinforcing Magnetorheological Fluids with Highly Anisotropic 2D Materials

Magnetorheological fluids (MRF) are suspensions of magnetic particles that solidify in the presence of a magnetic field. While non‐magnetic additives could improve MRF performance, explorations into such additives have not coalesced into an understanding of their influence, and particularly the role of additive morphology. Here, we explore α‐Ni(OH)2 2D sheets, with aspect ratios of ∼25,000, as highly anisotropic MRF additives. Experiments studying pressure‐driven flow of an MRF with and without these sheets show that their addition can increase the saturation pressure by as much as 46 %. However, shear‐mode rheology reveals that they can also weaken the MRF by inhibiting the chaining of the iron particles at low field strengths and have no effect at higher field strengths. In order to reconcile the strikingly different results, we propose that 2D materials introduce a non‐Newtonian handle to modify smart fluids in a manner that depends on the curvature of the shearing strain rate profile. Specifically, we identify a modification to the Buckingham‐Reiner model of pressure‐driven flow for a Bingham plastic in which the sheets widen the solidified plug. This work highlights the subtle interaction between particles in smart fluids and flows while emphasizing the opportunity for using anisotropy to tune this interaction. Highly anisotropic 2D sheets are explored as additives in magnetorheological fluids. The 2D sheets have little effect on the magnetic‐field‐dependent yield stress measured in boundary‐driven flow but increase the magnetic‐field‐induced pressure drop by up to 46 % in pressure‐driven flow. The authors explain this curious difference by proposing that the 2D sheets reinforce the magnetically solidified plug in pressure‐driven flow.